Construction of a rolling-bearing unit for wheel support, as disclosed in Japanese patent publication No. Tokukai 2001-221243, is shown in FIGS. 10 and 11. First, a first example of this construction as shown in FIG. 10 will be explained. A wheel rim 1 is supported by the rolling-bearing unit 2 for wheel support on the end section of the wheel axle 3 of the suspension apparatus such that it can rotate freely. In other words, the inner races 5, 5 of the rolling-bearing unit 2 for wheel support are stationary races and fit around the outer surface of the support shaft 4 that is fastened to the end of the wheel axle 3, and are fastened by a nut 6. On the other hand, the wheel rim 1 is connected and fastened to the hub 7 of the rolling-bearing unit 2 for wheel support, which is a rotating race, by a plurality of studs 8, 8 and nuts 9, 9.
A double row of outer-race tracks 10a, 10b, which are both rotating race tracks, are formed around the inner peripheral surface of the hub 7, and an installation flange 11 is formed around the outer peripheral surface of the hub 7. The wheel rim 1, together with the drum 12 of the braking apparatus, is connected to and fastened to the surface on one side of the installation flange 11 (outside surface in the example in the figure) by the studs 8, 8 and nuts 9, 9.
A plurality of rolling bodies or balls 14, 14 are located between the outer-race tracks 10a, 10b and the inner-race tracks 13, 13, which are stationary race tracks and are formed around the outer peripheral surface of the inner races 5, 5, and are held by retainers 15, 15 such that they can roll freely. By combining all of these components in this way, a back-to-back double-row angular ball bearing is formed, and it supports the hub 7 around the inner races 5, 5 such that it rotates freely, and supports radial load and thrust loads. Seal rings 16a, 16b are located between the inner peripheral surfaces on both ends of the hub 7 and the outer peripheral surface of the end sections of the inner races 5, 5, and they seal off the space where the balls 14, 14 are located from the outside space. Furthermore, a cap 17 covers the opening sections on the outside end of the hub 7 (throughout this disclosure, the outside in the width direction when the bearing is installed in the vehicle is referred to as the outside in the axial direction, and similarly, the center in the width direction is referred to as the inside).
As shown in FIG. 10, when using the rolling-bearing unit 2 for wheel support described above, the support shaft 4, around which the inner races 5, 5 are fastened, is fastened to the wheel axle 3, and the wheel rim 1, combined with the tire (not shown in the figure), and the drum 12 are fastened to the installation flange 11 of the hub 7. Also, of these, the drum 12, combined with the wheel cylinder and shoes (not shown in the figure) that are supported by the backing plate that is fastened to the end of the wheel axle 3, form a drum brake for braking. During braking, a pair of shoes that are located on the inner-diameter side of the drum 12 press against the inner surface of the drum 12.
Next, a second example of prior construction as shown in FIG. 11 is explained. In the case of this rolling-bearing unit 2a for wheel support, the hub 7a, which is the rotating race, is supported on the inner-diameter side of the outer race 19, which is a stationary race, by a plurality of rolling bodies or balls 14, 14 such that it rotates freely. In order for this, a double row of outer-race tracks 10a, 10b, which are stationary race tracks, are formed around the inner peripheral surface of the outer race 19, and a first and second inner race track 20, 21, which are rotating race tracks, are formed around the outer peripheral surface of the hub 7a. This hub 7a comprises a hub member 22 and inner race 23. Of these, an installation flange 11a for supporting the wheel is located on the outside end section of the outer peripheral surface of the hub member 22, and similarly, a first inner-race track 20 is formed in the middle section, and also similarly, a small-diameter section 24, which has a smaller diameter than the section where the first inner-race track 20 is formed, is formed in the middle section near the inside end of the hub 7a. Moreover, an inner race 23, around whose outer peripheral surface there is a second inner-race track 21 having an arc shaped cross section, fits around this small-diameter section 24. Furthermore, the surface on the inside end of the inner race 23 is held in place by a crimped section 25 that is formed by plastically deforming the inside end section of the hub member 22 outward in the radial direction, and this fastens this inner race 23 to the hub member 22. Also, seal rings 16c, 16d are located between the inner peripheral surface on both ends of the outer race 19, and the outer peripheral surface in the middle section of the hub 7a and the outer peripheral surface of the inside end of the inner race 23, and they seal off the space between the inner peripheral surface of the outer race 19 and the outer peripheral surface of the hub 7a where the balls 14, 14 are located from the outside space.
In the case of the rolling-bearing unit for wheel support described above, an increase in the torque (rotation resistance of the rolling-bearing unit for wheel support) required to rotate the hub 7 (or 7a) due to the existence of the seal rings 16a, 16b (or 16c, 16d), which seal off the openings on both ends of the internal space where the balls 14, 14 are located, is unavoidable. As a result, the driving performance, centered on the acceleration performance and fuel consumption of the automobile in which the rolling-bearing unit for wheel support is installed, becomes poor, so in the recent trend for more energy efficiency, improvement is desired.
Construction for reducing the resistance of the section of the sealing rings, and reducing the rotation torque of the rolling bearing has been considered in the past, such as the construction disclosed in Japanese Patent Publication No. Tokukai Hei 10-252762 in which the design of the interference of the seal lip is considered, and also the internal design such as the type of bearing, amount of pre-loading, shape of the components, contact angle or the radius of curvature of the race track surfaces, or also the type of grease used, and shape and material of the seal rings, and the like have all been considered. However, performing design work that correlates and properly regulates all of these elements, maintains the necessary seal performance and reduces the aforementioned rotation torque is troublesome. Therefore, simpler construction that is capable of reducing the rotation torque of a rolling-bearing unit for wheel support is desired.
The object of the rolling-bearing unit for wheel support of this invention is to solve the problems described above.